Toll switching telephone system



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TOLL SWITCHING TELEPHONE SYSTEM Filed Aug. 25, 195o 12 sheets-sheet 7 TWO WAY TRUNK 2 RETAR 724 \733 I IR? I R740 I RING AUTO. R.c.o. 79

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TOLL SWITCHING TELEPHONE SYSTEM Filed Aug. 25, 1950 l2 Sheets-Sheet 9 9l4 TWO WAY TRUNK 2l0 JNVENToR. F IG. 9 Harvey W Balzer Wag/,M

Dec. 9, 1952 H. w. BALzl-:R

TOLL. SWITCHING TELEPHONE SYSTEM 12 Sheets-Sheetl l2 Filed Aug. 25, 1950 Patented Dec. 9, 15952 UNITED STATES PATENT OFFICE TLL SWITCHING TELEPHONE SYSTEM Harvey W. Balzer, Downers Grove, Ill., assignor to Automatic Electric Laboratories, Inc., Chicago, Ell., a. corporation of Delaware Application August 25, 1950, Serial No. 181,509

36 Claims. 1

The present invention relates to multi-cnice toll switching telephone systems in general and more particularly to improved trunking arrangements and apparatus for interconnecting the various exchanges in such systems.

The invention is particularly applicable to a toll network including at least two widely separated automatic telephone exchanges having toll switchboards of the type disclosed in the copending Balzer application, Serial No. 181,508, iiled August 25, 1950. Each of the above eX- changes may be geographically located in the toll switching system so that they respectively serve as central toll switching points for the nearby telephone exchanges and so that they also serve as tandem switching points for through toll connections.

In handling toll calls received at the toll switching centers the switching apparatus thereat may be controlled over incoming toll lines to route such calls directly to the local exchange subscribers, or directly to other nearby exchanges, or to an idle operator position at the dial toll switchboard in the associ-ated switching center. In the latter event, the answering operator will control the switching apparatus of the network to complete the connections received thereat to the desired called destination. The dial toll switchboards at the respective toll switching centers each include several operator positions which may be divided into one or more groups to handle different classes oi toll calls. Consequently, certain of the positions may be selectively conditioned to handle inward and ring-down toll calls, certain other positions may be selectively conditioned to handle CLR toll calls and the remaining positions may be selectively conditioned to handle TX toll calls. As an alternative arrangement, each of the positions of the switchboard may be selectively conditioned to handle all three classes of toll calls so that each position functions as a universal position to handle inward, CLR and TX toll calls.

In toll switching system of the type briefly described above, it has been necessary heretofore to stati the switchboards provided in the two remotely located toll switching centers with one or more operators twenty-four hours a day. However, during the light trailic periods, all of the toll calls, as well as special service calls, may easily be handled by an operator at the toll switchboard in only one oi the toll centers.

Accordingly, one of the main objects ofthe present invention is to provide in a toll switching system of the character described, improved trunking facilities whereby the two-way toll lines interconnecting the two toll switching centers may be used for normal two-way toll traffic between the two switching centers during heavy traliic periods and whereby the same two-way toll lines may be utilized to automatically route all toll calls and special service calls intended for the toll switchboard and special service position at one of the toll switching centers'to the toll switchboard at the other switching center during light traic periods. With this arrangement, it is not necessary to stai any of the positions of the toll switchboard at one of the toll centers during light trailic periods.

It is another object of the invention to provide in a toll switching system of the character described improved trunk circuits terminating vthe toll lines interconnecting the toll switching centers which are arranged to discriminate between the regular toll calls directly routed thereover during the heavy trailic periods and the diierent classes of toll calls, intended for an unstaiied toll switchboard at one of the toll centers,.auto matically transferred thereover to the staffed other toll switchboard at the toll center during light traffic periods.

It is still another object of the invention to provide in a toll switching system of the character described improved trunk circuits terminating the toll lines interconnecting the toll centers which are arranged to transmit different identifying signals indicative of the different classes of toll calls automatically transferred thereover when` one of the toll center switchboards is unstaffed.

It is a still further object of the invention to provide in a toll switching system of the charf acter described improved trunk circuits terminating the toll lines interconnecting the toll centers which are arranged for dial-back operation so the toll calls automatically transferred thereover to an operator at the staffed ltoll switchboard in one of the toll switching centers may be completed therefrom to the called destination by controlling the switching apparatus in the other toll switching center over the same trunk circuits and toll line utilized inautomati-ll cally transferring the connection to the staffed position.

It is a further object of the invention to proide in a toll switching system of the character described, an improved transfer allotter for automatically routing the dii-ferent classes of toll calls, intended for an unstaffed position of the toll switchboard at one toll center to an attended position of the toll center in a predetermined` preferential order.

It is still another object of the invention to provide in a toll switching system of the character described an improved concentration switching unit for automatically routing both local and toll special service calls intended for the unstaned special service position at one ci the toll centers over an idle one of a plurality of two-way toll lines to a staffed position ci the toll switchboard at the other toll center.

Description of the drawings The invention, both as to its organization and method of operation, together with further cbjects and advantages thereof, will best be understood by reference to the following specication taken in connection with the accompanying drawings, in which Figs. i to 12, inclusive, when assembled in the manner illustrated in Fig. 13, illustrate the details of atoll switching telephone system having incorporated therein the features of the invention briefly outlined above.

Referring now to the drawings, it will be seen that the toll switching system illustrated therein comprises a central cince |00, a distant manual cnice 200, a distant automatic exchange 2533 and a distant toll center 2&6. All of the above noted exchanges are interconnected by means oi' the usual toll lines and are arranged so that connections may be established between the various exchanges either directly or through one or more tandem exchanges. The automatic switching apparatus in the central cnice m8, which is schematically illustrated in Figs. l and 2, is utilized in extending toll connections from the other exchanges in the system to the automatic dial toll board in the central cnice Ill and to the subscribers thereat. Also, the switching apparatus in the central cnice im] is utilized in extending connections from the dial toll board thereat to the other` exchanges in the system and in automatically transferring connections intended for the toll board thereat to the toll board at the distant automatic exchange N33, as will be described hereinafter. The distant manual cnice 200 serves a plurality of manual subscriber substations and is connected to the central cnice lull by means of a plurality of two-Way toll lines, such as the toll line 2li. Thus the distant manual cnice 2t!! and the central cnice IUD are interconnected over two-way toll lines for handling toll tranic between the two onices. The distant automatic exchange 293 serves a plurality of local subscriber substations and is connected to the central omce IDB by means of two-way toll lines, such as the toll line 2I2. Each toll line terminates in the respective exchanges in two-way trunks, such as the trunks 2I0 and 204. The distant automatic exchange 283 is also provided with an automatic dial toll board which is substantially the same as the toll board provided in the central cnice |08. At this point, it may be well to mention that the toll line 212 is accessible in either of the two exchanges by the operators at the respective dial toll boards and by the local subscribers in the respective exchanges. The distant toll center 236 serves a plurality ci automatic subscriber substations and is connected to the central cnice by means of a plurality of 'toll lines, such as the toll line 2l3, which are utilized for handling two-way toll tranic between the two exchanges. In addition to the foregoing, the distant toll center 296 is provided with a toll board 201 which may be of the type normally employed in telephone exchanges of the Bell system. From the foregoing, it will be understood that the exchanges 260, 253 and EBS are respectively connected by means of toll lines to the central cnice it and that toll connections may be established between any two of the above noted exchanges by means of the switching apparatus provided in the system and, if necessary, with the assistance of the toll operators at the toll boards in the exchanges.

Referring now to Fig. 2 of the drawings, it should be understood that the dial toll switchboard in the central ofnce ille includes a plurality of operator positions such as the operator position 22d@ and each position includes a plurality of link circuits, such as the link lt. It should also be understood that each operator position may be selectively conditioned to handle one or more of three different classes ci toll service or may be selectively conditioned to handle all three classes of toll service. More specically, each operator position, when selectively conditioned for universal operation may answer connections received over the CLR trunks, such as the trunks ill and SMA, from the local subscribers in the central cnice lll; it may also handle toll calls received over the inward and toll ring-down trunks, such as the trunks 33S and 36B, from the distant exchanges, and it may also handle TX calls which are received over the TX trunks, such as the trunk lidll. Each of the trunks noted above are provided with link nnders, such as "Idil, Till, del, 9&8 and HB9 which are governed in accordance with the trunk distributors ItA, IBQB and lSilBC to distribute the calls to the operator position 22953 by means of the links associated with the position. More specically the CLR trunk distributor wenn is arranged to distribute the CLR calls received on the CLR trunks SSG and EiiA by way of the link nders lill! and 'lill and the link ll to the operator position 22ml. Also, the inward trunk distributor lllB is arranged to distribute both inward toll and ring-down toll calls received on the inward trunk Sile and the toll ring-down trunk 8nd by way of the link nnders Sill and 98B and the link i3d!! to the operator position 228D. Finally, the TX trunk distributor liillC is arranged to distribute TX calls received on the TX trunk Indi) via the link nder lldil and the link It to the operator position 22nd.

The various links at the operator position 220i) are directly connected to toll preselectcrs, such as the toll preselector Utili), to enable the operator thereat to extend calls received at her position to any of the other exchanges in the toll switching system and also to the local subscribers in the central oince lllil. For this purpose each operator position is provided with the usual impulse sender for controlling the operation of the automatic switching apparatus provided in the toll switching network. The switching apparatus included in the distant automatic exchange 2l3 comprises a local switch train l2@ or" the Strowger type which is utilized by the local subscribers to complete local connections and which is also utilized to complete toll connections. lThe toll switch train in the automatic exchange 2% includes the toll selectors itl and R22 and the toll preselectcr 123. These selectors are employed in either extending toll connections from the automatic exchange 2%3 to the central cnice mi! or in completing toll connections, received from the central cnice lll@ to the local subscribers or to the dial toll board thereat. Finally, the distant automatic exchange 203 includes a dial toll board having a plurality of operator positions, such as the position |25, a plurality of link circuits, such as the link |20, and a plurality of trunk distributors, such as the trunk distributor |27. This dial toll board is substantially the same as the dial toll board provided in the central oice and described in detail in the previously mentioned Balzer application.

Fig. 3 of the drawings discloses the details of a concentration unit 205-which is utilized in transferring both local and toll information calls, intended for the information operator at the central ofce |00 to the operator at the dial toll board in the distant automatic exchange 203. The concentration unit 2|5 is provided with a special service trunk finder |204 of the rotary type which is adapted to search for a calling information circuit, such as ||5 and H5. Also, the concentration unit 205 is provided with a trunk finder |205 which is utilized to search for an idle two-way trunk extending to the exchange 203 for the purpose of automatically transferring either a local or a toll information call, intended for the information operator position ||l in the central office |00 to the toll operator at the automatic exchange 203 whenever the information operator position l l0 is unstaifed.

Fig. 4 discloses the night transfer allctter |200 which is utilized to automatically transfer CLR calls, inward and ringdown toll calls and TX calls, intended for an operator position at the dial tcll board in the central office |00, to a toll operator at the dial toll board in the automatic exchange 203. The allotter |200 is rendered effective to perform the above mentioned automatic transfer operation only when the dial toll board at the central office |00 is unstaffed, for example, during the light traffic periods. With this arrangement, the Various classes of toll calls which are normally extended to the dial toll board in the central office |00 will be automatically routed over available interconnecting toll lines to the dial toll board in the automatic exchange 203.

Figs. 5, 6, and 7, disclose the details of a, twoway trunk circuit 200 provided in the distant automatic exchange 203 which terminates the toll line 2|2 extending to the central ofice |00. This trunk circuit is provided with a rotary type link nder 500 which is utilized to automatically extend night transfer calls received over the toll line 2|2 to an idle link, such as the link |25, at the operator position |25 of the dial toll board. The trunk 204 is also connected via the cable |30 to the inter-toll selector |2| so that the operator at the dial toll board in the central oice |00 may control the switching apparatus in the distant automatic exchange 203 to extend toll connections to the local subscribers thereat and, under certain conditions, to the operator position |25. Finally, the trunk 200 is connected to the banks of the toll selectors |22 and |23 via the cable |32 so that the operator at the operator position |25 may extend connections via the toll preselectcr |23 to the central oce |00.

Figs. 8 to 11, inclusive, disclose the details of the two-way trunk circuit 2|0 provided in the central oiice |00 which terminates the toll line 2|2 extending to the distant automatic exchange 203. The trunk circuit 2|0 is accessible from the banks of the toll preselector |000 via the cable ||8| so that the dial toll board operator at position 2200 may extend connections over the toll line 2|2 to the switching apparatus in the exchange 203 and it is connected via the cable H82 to the intertoll selector ||2 so that the dial toll board operator at the exchange 203 may extend connections over the toll line 2 2 to the switching apparatus in the central oce |00. Finally, the two-way trunk 2| 0 is accessible over the cable 382 to the link finders |00, |0|, 30|, 900 and H00 and the trunk finder |205 for the purpose of automatically transferring calls, intended for the central omce toll operator and the central cnice information operator, over the toll line 2|2 to the dial toll board operator in the distant automatic exchange 203.

Fig. 12 discloses the details of the toll informatien circuit H5 and the local information circuit H0 which are utilized respectively to extend information calls to the information operator position H4 provided in the central oiice 00. During night service the information operator position H0 may be unstaffed as previously noted thereby to control the toll and the local information circuits ||5 and HS so that the information calls normally answered at the position H0 will be automatically transferred Via the concentration units 2|5 or 2 |5A and the two-way toll trunks 2 I0 and 20B to the dial toll board operator in the automatic exchange 203.

A detailed description of the operation of the toll switching telephone system in extending toll calls between the central office |00 in one t'oll switching center and the distant automatic exchange 203 in another toll switching center will now be given.

Toll call from the central oce operator position 2200 to the distant automatic exchange 203 Before describing the operations whereby the toll operator at position 2200 controls the switching apparatus to extend a connection to the distant automatic exchange 203, a brief description will be given of the operation of the switching apparatus in the central oice |00 whereby calls are extended to the operator position 2200 for completion. For this purpose, it will be assumed that the subscriber at substation Ti desires tc extend a connection to the distant automatic exchange 203 and has `dialed appropriate digits to control the switching apparatus in the central office |00 to extend a call to the operator position 2200. This connection is extended over a path including the line circuit |00, the line switch |07, the local selector |03, the trunk line 000 and the CLR trunk 000. As soon as the connection is extended to the CLR trunk 000, the CLR trunk distributor |5005. controls the link nder 700 to search for and seize an idle link, such as the link |300, associated with the operator position 2200. The operator thereat may now answer the call and ascertain the destination thereof from the calling subscriber at substation T|.

Toll calls which originate in a local sub-'office in the area served by the central office |00 may be extended to the operator position 2200 by way `of the local rst selector |03 and the trunk line 680A to the CLR trunk 600A. In response to such a call, the CLR trunk distributor |SO0A controls the link finder 10| to search for and find an idle link such as the link |300, and thus extend the connection to the operator positionV `2200.

The CLR trunk circuits 00|) and 600A are substantially identical, but they are separately grouped so that they are utilized respectively to extend local subscriber toll calls and subexch-ange toll calls to the dial toll board in the central offlee |00.

By grouping the CLR calls in the m'anner noted above, the operator at position 2200 receives a.

signal indicative of the particular vcalling group of CLR trunks.

A third class of toll call which may be extended to the operator position 220i! includes calls received over either the inw-ard toll trunk 39S or the ring-down toll trunk 809. The inward toll trunks are utilized to extend toll `connections from distant automatic exchanges and the ringdown toll trunk-s are utilized to extend calls from distant manual exchanges. Calls received over these trunks, however', are classified as inward toll calls and the operator at position 22% handles such calls ras inward toll calls whether they are received over the inward trunk or the ringdown trunk. A call originated at the distant toll center 2&5, either at a subscriber substation or at the toll board 2t?, may be extended via the repeater l'A, the interconnecting toll line 2l3 and the intertoll trunk 4de, to the intertoll selector iii. Calls received over the above described path may vbe extended froin the intertoll selector Htl, to the local subscribers in the centrai office los by way of the local switch train or they may be extended via the intertoll ring control circuit lil, the intertoll selector H2 and the trunk line 3Q@ to the inward toll trunk 336i. A call extended via the local switch train to local subscribers in the central office is@ does not require the 'assistance of the operator at the operator position 225e. However, if a toil call is extended to the inward toll trunk 3dS, the inward trunk distributor 5655 controls the link inder to search for and iind an idle link, such as the link 438i), and thus extends the connection to the operator position 22MB. The operator thereat upon ascertaining the destination of the particular toll call may complete the connection in the manner to be described hereinafter.

When the manual operator at the toll switchboard Ziii originates a toll call, the ring-down toll trunk istil is seized over the interconnecting toll line 2i i. In response to the seizure of the `ring-down toll trunk 8Go, the inward trunk distributor 968GB controls the link finder Seil to Search for and nd an idle link, such as the link iiil, and thus extend-S the toll call to the operator position 22ill. After ascertaining the destination of the received toll call the toll operator at position 22de may further extend the connection in the manner to be described hereinafter.

In the above brief description, it was assumed that the toll calls from the distant toll center 22d 'were extended to the operator position 22de via the inward toll trunk However, under certain conditions, the operator at the switchboard in the distant toll center Ei may desire to extend the call to the operator position 225i@ via the TX trunk lotti thereby to indicate the fact that the central ofiice toll operator must irst locate the desired called subscriber and then recali the operator at the distant toll center 203. TEE- calls are extended from the distant toll center Edil via the repeater 'A, the interconnecting toll line 2i3, the intertoll trunk the intertoll selector and the trunk line iti, to the TX trunk in response to the extension of the call to the TX trunk it@ the TX vtrunk distributor iiitC controls the link finder liti] Ito search for and find an idle link, such as the link i399 and thus extends the connection to the operator position 22M. Upon ascertaining the destination of the call received at her position and the information regarding the number of the call- 8 ing subscriber the operator at position 2280 will release the connection and then attempt to locate the called subscriber before recalling the calling subscriber.

In view of the foregoing brief description of the manner in which toll calls are extended to the central office toll operator position 220D, it will be understood that all such toll calls must be completed to the desired called destination under control of the operator at position 22M. For a detailed description of the operation of the switching apparatus involved in the above described toll calls, including the details of the link i363 and the operator position equipment 2280, reference may be had to the previously mentioned Balzer application.

In order to describe the operations of the twoway trunk circuits 2M and 2i@ interconnecting the toll centers including respectively the distant automatic exchange 2&3 and the central office its, it will be assumed that one of the above described toll calls received at the operator position 225e in the central oiice iii?) is to be extended to the distant automatic exchange 2&3. Accordingly, the operator at position 2293 actuates the keyset and impulse sender (not shown) associated with her position in accordance with the digits necessary to extend the call to the desired called party. Thereafter, the impulse sender functions, in the manner described in the above mentioned Balzer application, to control the link i390 included in the connection and the toll preselector M59. The toll preselector Hill!! responds to the impulses of the first digit transmitted by the impulse sender to raise its Wipers opposite the level terminating a group of trunks including the trunk line extending to the two-way trunk circuit 2id. Thereafter, the toll preselector M630 will automatically rotate its wipers over the contacts in the banks oi the selected level to search for and nd a trunk connected to an idle two-way trunk circuit such as the two-way trunk circuit 2H).

Referring now to Figs. 8 to ll, inclusive, it will be seen that the two-way trunk circuit 210 is accessible to the toll preselector M390 via the conductors Cli83 to Cllii, inclusive, included in the cable i ESI. The circuit is marked idle to the toll pr selector Hill@ by battery potential connected to the C conductor Cl S5 by way of the winding of the outgoing seizure relay R920. Consequently, when the toll preselector M50 seizes the two-way trunk 2 i ii, it forwards ground potential over the C conductor Ci |85 in order to operate the relay RZ and it forwards battery potential over the EC conductor C1135 in order to operate the pulse-out relay R958 over a circuit including its low resistance lower winding and ground at contacts 9N. In view of the foregoing, it will be understood that the relays R920 and R553 are operated as soon as the toll preselector Vidi! seizes the two-way trunk 21B. As soon as the relay R operates, at its contacts 924 and 92E, it applies ground potential to the C-toll conductor C292 and the C-local conductor C29! thereby to mark the two-way trunk 250 busy to night transfer' calls.

When the pulse-out relay R operates it completes at its contacts 962 an obvious circuit for operating the outgoing hold relay R950 and the latter relay, at its contacts 95|, disconnects the line termination network, including the condenser 995 and the resistor 994, from the and line conductors Ci i851 and CHM. As a further result of the operation of relay R960, at its contacts 96|, it connects battery potential through the winding of the stop dial control relay R940 to the dial conductor C883 extending to the twoway trunk 200 in the exchange 208. This circuit is involved in various operations between the two-way trunks 200 and 2I0 and may be traced from battery by way of the winding of relay R940, the contacts 992, 98E, 922 and |032, the dial conductor C883 in the cable 8|3 extending to Fig. 8, and the midpoint of the lower windings of the repeating coil 89|. At this point, the circuit including the dial conductor C883 is extended in parallel over the line conductors C795 and C798 of the toll line 2|2 to the lower windings of the repeating coil 79| associated with the two-way trunk 209. The circuit is further extended from the mid-point of the lower windings of the repeating coil 79| via the dial conductor C704, the contacts 593, 57|, 55|, 52| and 803 and the windings of the marginal booster battery relay R7|0 and the pulse-in relay R989, to ground. In this circuit, the relays R888, R7|8 and R909 are connected in series but only the pulse-in relay R880 and the dial relay R980 operate. Due to the resistance of the above mentioned relays, the marginal relay R7 I0 does not operate at this time. It may be well to mention, however, that the marginal booster battery relay R7|0 will operate in series with the pulse-in relay R880 only in the event a pulse of negative booster battery is transmitted overv the dial conductor C794 from the two-way trunk 2 I9. Y

Referring now to the relay R909, it will be noted that when the relay operates over the above traced circuit, it completes, at is contacts 942, an obvious circuit for energizing the lower winding of the outgoing hold slave relay R930. The relay R980 now operates and locks itself in its operated position via its contacts 935 to ground at contacts 953. The two-way trunk El@ is now in condition to repeat the subsequent impulses received from the impulse sender over the EC conductor CI |88 to the pulse-in relay R088 in the two-way trunk 200.

Referring now to the pulse-in relay R880 it will be seen that as soon as the relay operates over the above traced circuit including the dial conductor C799, it completes, at its contacts 88|, a circuit for operating the incoming hold relay R590. ground potential via its contacts 999 and the contacts 772 to the C conductor C787 in the cable |32 in order to mark the two-way trunk 208 busy in the banks of the toll selectors |22 and |23 having access thereto. Also, at its contacts 898, the relay R090 connects battery potential by way of the winding of the marginal selector seize relay R870 and the contacts 883 to the EC conductor C780 in the cable |30 extending to the toll selector IEI. Simultaneously therewith, as it contacts 690, the relay R580 connects ground potential to the C conductor C783 in the cable 38 extending to the toll selector IEI. The application of ground potenti-al to the C conductor C788 seizes the toll selector 52| and in response thereto the toll selector returns va low resistance ground potential over the EC conductor C784 in order to operate the marginal selector seize relay R'970. In view of the foregoing, it will be understood that when the pulse-in relay R680 operates over the dial conductor C790, it causes relays R898 and R670 to operate in the order named and it also causes ythe seizure of the toll selector |2| to prepare the same to respond to The relay R900 upon operating, applies r the relay R900 in the two-way trunk 2|0.

10 subsequent impulses transmitted over the dial conductor C794.

DIALING The impulse sender at the operator position 2280 now transmits the impulses corresponding to the remaining digits of the called number over the connection including the link |300, the toll preselector |800, the EC conductor C||86 and The pulse-outrelay R960 momentarily restores in response to each impulse and, at its contacts 98|, interrupts the dialing circuit including the relay R900 and the relay R680. In this manner, the

' pulse-in relay R880 follows the impulses repeated thereto by the relay R960 in order to control the toll selector |2| and the switches in the local switch .train |20 and thereby complete the connection to the desired called subscriber.

The first time the pulse-out relay R980 restores, i. e., in response to the rst impulse of each series of impulses transmitted thereto, it completes a circuit via the contacts 983, 952 and 978 for operating the slow-to-release control relay R988. Due to the slow-to-release characteristics of relays R950 and R980, these relays remain in their operated positions during each series of impulses.V However, during the interdigital pause between successive digits the relay R980 restores and the relay R950 remains in its operated position. As soon as the relay R980 operates, it completes, at its rcontacts 98|, an obvious circuit for energizing the lower winding of the automatic ring cut-off relay R|020. The relay RI820 now operates and, at its contacts I02| and |023, it disconnects the ring relay R|0|0 and the resistor |0|5 from the line conductors CI |83 and CI |88. The relay R|020 is arranged to disconnect the ring relay R|0I0 from the calling line during the dialing of each digit and for a short interval thereafter, in order to prevent an automatic ringing control signal from being repeated to the distant automatic exchange 203 after the dialing of the last digit.

As a further result of the operation of the relay R880, at its contacts 982, it connects battery potential via the resistor 984 to the dial conductor C889 in multiple with the battery potential 'applied thereto through the winding of the dial control relay R990. With this arrangement, the impedance of the dialing circuit is lowered in order thereby to improve the pulsing vcircuit for controlling the pulse-in relay R880 in the twoway trunk 200. Finally, at its contacts 983, thel relay R980 opens a point in the incomplete circuit for the supervisory control relay R970 in order to prevent its operation when the relay R900 momentarily restores during impulsing.

After each series of impulses, the pulse-out relay R980 is retained in its operated position and, at its contacts 903, opens the circuit for the control relay R980 which slowly restores to normal. As soon as the relay R980 restores, it interrupts, at its cont-act 98|, the previously described circuit for operating the slow-to-release relay R|020. This relay, due to its slow-to-release characteristics, delays restoration long enough to prevent the` automatic ring control from being transmitted to the ring relay Referring now to the pulse-in relay R680 in the two-way trunk 200, it will be understood that this relay restores to normal, in synchronism with the relay R940 in the two-way trunk 2|0, each time the dialing circuit is interrupted at the contacts 96| by the pulse-out relay R960. Each time the relay R685 restores to normal, it cornpletes, at its contacts 682, a circuit ior energizing the lower winding oi the supervisory control relay Rb'ou in series with the operated marginal relay Rolli. rlhis circuit may be traced from ground via the contacts |522, o i5, ii, 69:5 and cl2, the lower winding of the relay R660 and the winding of the relay Rl, to battery. The relay Roco now operates and, at its contacts 557, completes a locking circuit for itself which includes its upper winding and the contacts 523, 661 and Seil. 'l'hus it will be understood that in response to the rirst impulse transmitted to the puise-in relay R580, the relay R650 will operate and remain in its operated position until the connection is released. The incoming hold relay R690 due to its slow-to-release characteristics remains in its operated position during impulsing.

As a rui-ther result of the operation of the relay Rru, at its contacts 6:51, it short-circuits the winding of the marginal relay R111] thereby to reduce the resistance i' the impulsing circuit including the dial conductor C154 and thus improve the impulses transmitted thereovei.

Finally, at its contacts 68S,` the pulse-in relay Ril interrupts the EC conductor C184 extending to the toll selector 121 in order to repeat thereto the impulses of the various digits transmitted over the dial conductor Visits. Consequently, the rst series of impulses repeated over the EC conductor C784 by the pulse-in relay R'iiti controls the toll selector 12! to raise its wipers in a vertical direction a number of steps corresponding to the number of impulses included in the digit. Thereafter, the toll selector 121 automatically rotates its wipers over the selected level to select an idle trunk extending to the local switch train 121. Subsequent series of impulses corresponding to the remaining digits transmitted from the impulse sender at the toll operator position 2200 will control the local switch train |28 in a conventional manner to complete the connection to the desired called subscriber line.

Before describing the operations which result from the answering of the call by the called subscriber, a description will be given of the operations which result when a stop dialing condition is encountered by one of the switches in the switch train in the distant automatic exchange sfroiJ DIALING Ii during the setting up of the connection via the incoming switch train including the toll selector 121 and the local switch train 120, a stopdialing condition is encountered, the low resistance ground potential returned over the EC conductor C784 will be replaced by a high resistance ground potential and cause the marginal relay REW to restore to normal. Since the supervisory control relay R600 and the incoming hold relay R690 are in their operated positions during pulsing, the relay R610 upon restoring completes a circuit for operating the stop-dial relay RSM. This circuit may be traced from battery by way of the winding of the relay R640, the contacts 664, 0'11, 691, '124 and 590, to ground. As soon as the stop-dial relay R640 operates, it closes its X contacts 642 in order to complete a holding circuit for the pulse-in relay R680 from battery through the resistor 558. As a further result of the operation of the relay R640, at its contacts 543 and 6184, it transfers the dial conductor Clad from the previously traced circuit including ground potential through the winding of the pulse-ill relay R800 to a circuit including battery potential through the winding oi tile dial control relay Roo'u. when this transrer is made, a circuit is completed irol'n battery via the winding ol the relay Ramo, contacts u2, let, zizi, eel, :nl and :alla and the dial conductor Cieli and the previously traced circuit including the winding or tne relay Rueil, to battery` As soon as tne dial conductor Clues in the two-way trunk 2cd is translerred Irom ground potential to battery potential in the manner described above, 'the relay Rollo in the twoway trunk zelo restores to normal. As a resuit or the restoration of the relay R960, at its contacts oe3, it completes a circuit for operating the supervisory control relay Raw, which may ne traced irom ground via the contacts oeil, 8d3, tot and :its and the winding of relay Rolli, to battery. Upon operating, the relay Rs'lc, at its contacts am and uw', substitutes a high resistance ground potential I'or the low resistance ground potential oli tile mi; conductor Cl loc'. lvlore specincaily, at its contacts Hi, the relay Raie connects ground potential, via the high resistance upper winding and the low resistance lower winding or' the relay Roco, to the hC conductor Clldo. 'lne hign resistance ground potential returned on the conductor Ci 18o' controls the impulse sender at the operator position aco to bloei; rurther transmission ol impulses to the two-way trunk 210. 'lne manner in which the impulse sender is controlled to block further transmission or' impulses in respense to a stop-dial condition, i. e. the return oi' a high resistance ground potential in place of the low resistance ground potential on the EC control conductor thereat, is described in detail in the previously mentioned Balzer application.

When the stop-dialing condition is removed by the switch train in the distant automatic exchange rc3, the high resistance ground potential applied to the EC conductor C184 is replaced by a low resistance ground potential thereby to reoperate the marginal relay R010. As soon as the relay RElo is reoperated, it interrupts the circuit for the stop-dial relay RSM), at its contacts 671. The relay Rodi! now restores to normal and, at its contacts 433 and Ella, disconnects battery potential through the winding of the dial control relay R from the dial conductor C194 and it reconnects the ground potential through the winding of the pulse-in relay R680 thereto. Finally, at its contacts W2, the relay RMU interrupts the previously traced holding circuit for the pulse-in relay R680.

As soon as the ground potential through the winding of the pulse-in relay R580 is reapplied to the dial conductor C, the dial control relay RSl-l in the two-way trunk 210 is reoperated in series with the pulse-in relay R080 in the twoway trunk 204. When the relay R940 reoperates, it interrupts, at its contacts Slis, the previously described circuit for the supervisory control relay R90 which now restores to normal. The relay R9'l0 new disconnects, at its contacts 915, the high resistance upper winding of the pulse-out relay R from the EC conductor C1180 and, at its contacts 9M, it reconnects ground potential through the low resistance winding of the pulseout relay R060 to the EC conductor C1186. In response to the return of the low resistance ground potential, in place of the higli resistance ground potential, on the EC conductor C1 185, the impulse sender at the operator position 2200 is controlled, in the manner described in the above senate 13 Y mentioned Balzer application, to remove the stopdialing condition and continue the transmission of the remaining impulses corresponding to the digits of the called destination.

BLOCKING AUTOMATIC RINGING The impulse sender provided at the operator position 22ml is arranged to normally transmit an automatic ringing control signal immediately after the last digit of the called number has been transmitted. The present call, however, is routed to the automatic switching apparatus in the distant automatic exchange 23 which is arranged to automatically signal a desired called subscriber after the terminals of the called subscriber line have been selected. Consequently, it is necessary to provide facilities in the two-way trunk 2 lil for blocking the automatic ringing control signal transmitted thereto by the impulse sender at the operator position 220e.

As previously described, the automatic ring cutm off relay Rlli is operated under control of the control relay Ril and the latter relay, being of the slow-to-release type, remains operated during the transmission of each series of impulses, to the automatic exchange 263. Consequently, the relay RIMS is in its operated position for a short interval of time after the last impulse of the last digit has been transmitted by the impulse sender. Shortly thereafter, the impulse sender transmis a short pulse of negative battery potential over the established connection to the conductor Cl 83 in the cable i |8l. This pulse is further eatended via the upper winding of the retard coil H324, the contacts i622 and the upper winding of the automatic ring cut-off relay RIMS, to ground. Since this pulse is transmitted after the last digit and before the slow-to-release relay Rt has restored, or at least before the slow-to-release relay Ri has restored, the battery pulse on the conductor Cl 183 holds the relay Rli operated over a circuit including its upper winding. As soon as the battery pulse is removed by the impulse sender, the relay Rli slowly restores to normal and, at its contacts IGZ I, it reconnects the ringing relay Rii to the conductor CI it. In this manner, the automatic ring cut-off relay RIBZB prevents the relay Rliilii from responding to the automoti-c ringing control signal battery pulse and thus prevents the automatic ringing control signal from being repeated over the interconnecting toll line 2l? to the switching apparatus in the automatic exchange 263.

BUSY CONDITION During the setting up of the above connection, it was pointed out that the impulses transmitted over the toll line 212 are repeated by the two-way trunk Zl in order to control the toll selector 2i and the switches in the local switch train lill in order to complete the connection t the desired called subscriber. Before describing the operations which occur when the called subscriber answers the connection, a description will rst be given of the operations which occur as a result of a busy condition encountered by the switching apparatus during the setting up of the connection or a busy condition encountered on the called subscriber line. In either event, alternate high and low resistance ground pulses will be returned over the EC conductor C184 to cause the intermittent restoration and operation of the marginal selector seize relay R810. At this point, it may be well to mention the fact that the alternate high and low resistance ground pulses will be transmitted at the rate of impulses per minute in the" event that the switching apparatus in the automatic exchange 263 encounters an all-trunksbusy condition, and that the pulses will be transmitted at the rate of 60 impulses per minute in the event that the particular called subscriber line is busy when the connection is established therewith, Each time the relay Rl restores under control of the busy indicating pulses returned over the EC conductor Clll, it completes, at its contacts tl l, the previously described circuit for operating the stop-dial relay R666. Consequently, the relay Reti! will now operate and restore at the same rate as the relay Ri. Each time the relay REM) operates and restores, it momentarily completes, at its contacts Gii, the previously described circuit for retaining the pulsein relay R589 in its operated position and, at its contacts 643 and 6131i, it alternately connects battery potential through the winding of the dial control relay R565 and ground potential through the winding of the pulse-in relay R539 to the dial conductor Citt over the previously described circuit. In this manner, the relay Rfil alternately connects battery and ground potential to the dial conductor C' at the same rate at which the marginal relay Rll restores and reoperates and thus transmits a flashing busy signal at 60 or 120 impulses per minute, depending upon the type of busy encountered, to the two-way trunk. 2id in the central office Mii?.

Referring now to the dial control relay Rfi in the two-way trunk 2m, it will be recalled that this relay is normally held in its operated position from ground potential through the winding of the pulse-in, relay Rtii in the two-way trunk 25M. Consequently, each time the battery potential is connected to the dial conductor C734, the relay Rte will restore, and each time ground potential is reconnected thereto, the relay Rlil will reoperate. Accordingly, the relay RSM) restores and reopera'tes at the same rate as the relay RSM! in the two-Way trunk 2M. Each time the relay Rgfiii restores, 'it completes, at its contacts slit, the previously described circuit for operating the supervisory control relay Rti and each time the relay R913@ reoperates, it interrupts the circuit for the relay RQ'IS. Consequently, the relay Reli) follows the flashing busy impulses and, at its contacts Qld and 15, it alternately connects ground potential through the high resistance upper winding and the low resistance lower winding of the relay Rtl to the EC conductor Ci E86 extending to the operator position 2299. However, the pulse-out relay Ri'c remains in its operated position during the transmission of the :dashing busy signal over the conductor Citt?. The alternate high and low resistance ground pulses transmitted back overl the EC conductor C! i855 control the operators position equipment to give her a flashing busy signal indicative of either an all-trunls-busy condition or a called subscriber line busy condition encountered during the establishment of the connection previously described. The calling operator at position Zi may now release the connection and attempt to complete the same at a later time.

CALLED PARTY ANSWERS thereat. In response to answering the call, the

switching "apparatus in the automatic exchange 203 operates to substitute a steady high resistance ground potential for the low resistance ground potential applied to the EC conductor C'i. The steady high resistance ground potential returned over the conductor 18d causes the marginal relay Rt'iil to restore to normal and thus complete, at its contacts 515, the previously described circuit for operating the stop-dial rela-y Relic. Upon operating, the relay R643, at its contacts Se?, completes the holding circuit for the pulse-in relay RSS@ and, at its contacts and it disconnects ground potential through the winding of the relay from. the dial conductor and substitutes, in place thereof, battery potential through the winding of the dial control relay Rte.

The substitution or" the battery potential for the ground potential on the dial conductor Cl causes the dial control relay Rlflal in the two-way trunk Elli to resore to normal. 531%3, the relay Rte completes the previously described circuit for operating the supervisory control relay R921@ and the relay R910, in turn, at its contacts gli and ill, replaces the low resistance ground potential on the EC conductor Ci i de with a high resistance ground potential through the upper and lower windings ci relay In this manner, a steady high resistance ground` potential substituted for the steady low resistance ground potential on the EC conductor Ci lA S in order to give the calling operator at position ilt an oir hook supervisory signal indicating that the called subscriber has answered the connection.

The calling toll operator at position 22 may now interconnect the calling and called subscribers and simultaneously disconnect her position equipment from the established connection in the manner described in the previously mentioned Balzer application. The conversational. path which is now completed from the calling party through the central olice Ehi to 'the called subscriber line in the automatic exchange includes the switching apparatus in the central onlce ill over which the calling end of the cenl,

nection is completed to the link the toll preselector istil, the cable 318i extending to the two-way trunk 2 the and -lconductors Ci i83 and Ci the and -lconductors Ciii and C882 in the cable dit, the inductively coupled windings of the repeating co-il til I, the conductors Cl and Cil oi the toll line the inductively coupled windings or" the repeating coil i the w and -iconductors Cl and; C193, the and -lconductors Cll and C' in the cable i3d, the

toll first selector l2 i, the local switch train li, and the called subscriber at substation T2.

ln the above described connection, it was assumed that the toll selector 125 and the local switch train ii in the exchange were utilized in completing the connection to the called subscriber substation, but it should be understood that the connection could also be directed vla the toll selector i'Zi and a link, such as the link i263, to the operator position the dial toll board in the automatic exchange '2&3 in the manner described in the previously mentioned Balzer application. lt should be understood that the switching apparatus and trucking arrangement provided at the automatic exchange Zeil is substantially the same as that provided in the central cnice ISB.

sUPERVlsioN If the called subscriber at substation T2 desires to recall the toll operator at the operator At its contacts position 2200, the switchhook at the substation may be actuated one or more times in the usual manner in order to transmit a switchhook signal. Each time the switchhook is momentarily actuated, the switching apparatus included in the automatic exchange 203 operates to replace the high resistance ground potential on the EC conductor C184 with a low resistance ground potential in order to reoperate the marginal relay R510 in the two-Way trunk 204. It may be well to mention at this time that the marginal relay R610 was restored to normal, when the called subscriber answered the call, by the substitution of a high resistance ground potential for the low resistance ground potential on the EC conductor C184. Consequently, when the low resistance ground potential is returned over the EC conductord C184 to reoperate the marginal relay R610, the relays R640, R660, REBi) and R690 in the twoway trunk 204 are in their operated positions and battery potential is connected through the winding of the dial control relay R550 to the dial conductcr C194.

Upon operating under the influence of the switchhook supervisory signal, the relay R610 interrupts, at its contacts 61|, the circuit for the relay R'M which now restores to normal. Upon restoring, the relay R669 reconnects ground potential through the winding of the pulse-in relay RSSS to the dial conductor C194 and it disconnects battery potential through the winding of the relay R556 from the dial conductor C194.

In the two-Way trunk 2I0 the substitution of the ground potential for the battery potential on the dial conductor C194 recompletes the previously described circuit for operating the dial control relay R940. At its contacts S43, the relay R940 interrupts the circuit for the supervisory control relay R910 which relay now restores to normal and, at its contact 910 and 915, substitutes a low resistance ground potential for the high resistance ground potential applied through the windings of the pulse-out relay R960 to the EC conductor CI ISG. The substitution of the 10W resistance ground potential for the high resistance ground potential applied to the EC conductor Ci |86 controls the link circuit I300 associated with the operator position 2200 to give the operator thereat an on hook supervisory signal.

The above series of operations all result from the momentary actuation of the called subscriber switchhook to transmit on hook supervisory signal, should be noted that when the switchhook is released to an 01T hook position, the marginal relay R610 will restore to normal and reoperate the stop-dial relay R640 in the manner previously described. The latter relay, in turn, will again replace battery potential through the winding of the relay R560, for the ground potential, on to the dial conductor C194 in order to restore the relay R94@ in the two-Way trunk 210. The latter relay, at its contacts 943, recompletes the operating circuit for the relay R910 thereby to again replace the high resistance ground potential on the EC conductor Cl |86. The toll operator at the position 2260 may now control the associated link I300 to reconnect her headset to the established connection and converse with the called subscriber.

Under certain conditions the calling operator at position 22 50 in the central oiice |00 may nd it necessary to rering the called subscriber at substation T2 or the toll operator at position |25 in the distant automatic exchange 203 over an 

